1. Field of the Invention
This invention relates to track-seeking and track-following servo systems for magnetic disk files.
2. Description of the Prior Art
In order to read and write data on a magnetic disk file, the read/write heads must be positioned over the desired one of a plurality of concentric recording tracks. The minimum distance between tracks is determined by the accuracy of the head positioning servo mechanism and the ability of the read/write channel to function with offtrack error. Increasing the track density (tracks per inch) necessitates decreasing the positioning error.
Present state of the art high performance disk files use a single servo head and a dedicated servo disk surface on which position information is written. The data read/write heads are all mechanically linked to the servo head so that once the servo head is on track, the data heads should be on track. However, the mechanical linkage between the servo head and data heads is a source of large misalignment error. Misalignment of the servo and data heads has dynamic and static components, including thermal drift and tilt of the carriage assembly which carries the heads.
Study of the economics of future generation disk files indicates that a new method of servoing the data heads to the data tracks is needed. The error between the servo head and the data heads is every large and will be expensive to eliminate using present servoing techniques (if it is at all possible).
There are two basic approaches to achieving the higher track density goal. The more complex of these is the so-called imbedded servo, and the second is sectorized or sampled data servo. Imbedded servo is the name for a technique in which the servo position information is imbedded in or written with the data on each track. In embedded servo, the position information comes from the track it is desired to servo on. Imbedded servo implies the ability to read and write data on a track without erasing the position information, and the ability to read the position information while reading or writing data, a difficult task at best.
In the sector servo approach, the positioning information also comes from the track to be servoed on, but at the expense of losing some data space. The method involves writing small patches or sectors of position information between large portions of data, with each sector of position information being used to control the servo system during the subsequent data portion.
While the technique of sector servo is not as elegant as that of imbedded servo, many of the problems of its implementation have already been solved. Low capacity/low performance disk files are presently on the market which use sector servo, but due to the large number of sectors needed (wsting up to 50% of the data space), the sector servo approach has not been applied to high performance drives.